1. Field of the Invention
The present invention is particularly concerned with superconductor compositions and in particular those superconductor compositions which are known as Type II. Of the Type II compositions those which exhibit the highest transition temperature have a particular type of crystallographic structure known as A-15 or as the beta-wolfram structure. By employing the thin film sputtering techniques of this invention for producing the desired stoichiometry, superconducting transition temperatures as high as 22.3.degree. K have been observed in Nb.sub.3 Ge.
2. Description of the Prior Art
The most favorable crystal structure for high transition temperatures has been found to be the A-15 or the beta-wolfram structure. Among the many binary systems which exhibit this crystal structure is the niobium-germanium system. Examples of the beta-wolfram structure of niobium-germanium using standard metallurgical techniques were prepared and were found to have a transition temperature of about 6.degree. K. Through the use of the standard metallurgical techniques, it was discovered that such compositions were slightly off the stoichiometry from the desired composition of Nb.sub.3 Ge. Based on these and other considerations it was hypothesized that if the stoichiometric Nb.sub.3 Ge could be formed it would have a very high transition temperature. Some experimental verification for this hypothesis was found when it was shown that by splat cooling the molten material of the proper composition a more nearly stoichiometric compound was produced. When such material was tested for its superconductivity, transition temperatures of about 17.degree. K were found.
As a result of this type of early work and later theoretical considerations, a thesis has been presented which is presently gaining wide acceptance in that it is believed that all high transition temperature superconductors are inherently structurally unstable, that is, the same factors which would produce a high transition temperature in these compounds also produces structural instabilities. If this theory is accepted, it is apparent that materials which might be expected to have very high transition temperatures if they could be made to crystallize into a structure most conducive to the high temperature superconductivity will not normally do so using standard preparation techniques.
Based on this reasoning the achievement of high transition temperatures must be dependent on the development of new techniques capable of fabricating materials with the desired crystal structures in metastable form. If successful, these materials with metastable crystal structures could in some instances be expected to possess very high transition temperatures. This has lead to the investigation of various thin film techniques such as sputtering, evaporation, and chemical vapor deposition which in the past have demonstrated the ability to produce metastable structures. As a result, sputtering techniques to produce niobium-germanium have been reported with the maximum transition temperature of 16.5.degree. K thereby indicating that the material is close to the desired stoichiometry.
In U.S. Pat. No. 3,506,940 to E. Corenzwit et al, there is reported a method of critically annealing pseudo-binary compositions of niobium-germanium-aluminum which showed improved transition temperatures irrespective of the technique used for the formation of the pseudo-binary composition. However, the highest superconducting temperature reported therein was 20.1.degree. K.
By following the parameters of the present invention, the critical annealing step is not mandatory yet a higher transition temperature, namely 22.3.degree. K has been reproducibly obtained.